Because at present all superconductors must be super-cooled in a
coolant such as liquid nitrogen to become superconductors.
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Resistance decreases with the decrease of temperature.
Superconductors are made by lowering the temperature.
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Because refrigerating superconductors to the cryogenic temperatures needed by current ones is expensive, severely limiting the applications they are used in.
Metallic superconductors need cooling to the temperature of liquid helium.
Copper oxide ceramic superconductors need cooling to the temperature of liquid nitrogen.
Room temperature superconductors, if they exist, would need little or no cooling.
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In a way, all currently existing superconductors are "low-temperature", but some more so than others. The traditional superconductors work up to about 20 K (or minus 253 Centigrade); more recent "high-temperature superconductors" work up to 100 K or so. 100 K is still minus 173 Centigrade, but it is much "hotter" than the traditional superconductors. The new "high-temperature" superconductors apparently work different than the old-fashioned ones; at least, the theory that explains the traditional superconductors fails to explain how the new superconductors work.
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In superconductors, no electricity is wasted because there is no resistance to the flow of electrons. In conductors any electricity not used, is wasted.